The present invention relates to new compounds useful in assisting in the polymerization of monomers in a free radical polymerization that has living-type kinetics. Polymers made with the control agents and processes for polymerization are also included.
The use and mechanism of control agents for free radical polymerization is now generally known, see for example U.S. Pat. No. 6,153,705, WO98/01478, WO99/35177, WO99/31144, and WO98/58974, each of which is incorporated herein by reference. The compounds described in these references are generally referred to as chain transfer agents. These references generally disclose dithiocarbamate or xanthate control agents having heterocyclical moieties. See, for example, WO 99/31144 at pages 31-34.
One general problem is that at high monomer conversions (e.g., above 70%) the polymers made with these control agents have a very high molecular weight component, which leads to a polymer that generally does not have a mono-modal molecular weight distribution. Without wishing to be bound by any particular theory, it is believed that this high molecular weight component is created at high conversions due to the coupling of propagating polymer chains. This coupling creates chain ends that cannot continue to be polymerized and are effectively xe2x80x9cdeadxe2x80x9d toward further propagation. The presence of xe2x80x9cdeadxe2x80x9d chain is particularly troubling when forming block copolymers with this controlled free radical polymerization methodology because a typical procedure for block formation uses the sequential addition of different monomers. xe2x80x9cDeadxe2x80x9d chains cannot participate in block formation in this methodology. Thus, there is a desire to eliminate the formation of dead chains.
With some embodiments, gel permeation chromatography experiments reveal a bimodal molecular weight distribution of the polymers made with free radical polymerization chain transfer agents of the type disclosed in these references. For example, WO 99/31144 fails to distinguish between bimodal molecular weight distributions and those that may be otherwise; consider example 22, where a bimodal distribution is revealed for control agent (62). The implication from a bimodal molecular weight distribution is that a number of dead polymer chains exist in those polymer samples.
It has now been found that certain control agents eliminate the bimodal molecular weight distribution at high conversions, additionally providing a very narrow molecular weight distribution. These control agents are particularly useful for block polymer formation using a sequential addition of monomer methodology.
This invention provides control agents that effectively diminishes or eliminates a bimodal molecular weight distribution at monomer conversions greater than 70%. These control agents also provide a polymer with a very narrow molecular weight distribution, generally less than about 1.10. In general the control agents useful to create the polymers of this invention and useful in the processes of this invention may be represented by the following general formula: 
wherein R1 may be selected from the group consisting of optionally substituted hydrocarbyl and heteroatom-containing hydrocarbyl and each of R2, R3 and R4 is independently selected from the group consisting of hydride, optionally substituted hydrocarbyl and heteroatom-containing hydrocarbyl.
Other aspects of this invention include polymerization processes using these control agents and polymers that can be made with the control agents. In particular, the control agents of this invention provide living-type kinetics and as such allow for the preparation of desired products, including block polymers, star architectures, grafts and hyperbranched polymers.
Thus, it is an object of this invention to provide control agents for a living-type free radical polymerization process where the polymers made during the process do not have a bimodal molecular weight distribution at monomer conversions greater than about 70%.
It is another object of this invention to provide a system for free radical polymerization of monomers that employs living-type kinetics where the molecular weight distribution is less than about 1.10.
Further aspects and objects of this invention will be evident to those of skill in the art upon review of this specification.